Process for producing biopolymer nanoparticles

ABSTRACT

In a process for producing a biopolymer nanoparticles, biopolymer feedstock and a plasticizer are fed to a feed zone of an extruder and the biopolymer feedstock is processed using shear forces. A crosslinking agent is added to the extruder downstream of the feed zone. The process has a production rate of at least 1.0 metric tons per hour. The feedstock and the plasticizer are preferably added separately to the feed zone. The extruder may have single flight elements in the feed zone. The temperatures in the intermediate section of the extruder are preferably kept above 100 C. The screw configuration may include two or more steam seal sections. Shear forces in a first section of the extruder may be greater than shear forces in an adjacent downstream section of the first section. In a post reaction section, water may be added to improve die performance.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation application of application Ser. No. 12/377,501filed on Nov. 3, 2010, National Phase of PCT Application No.PCT/US2007/075901 filed on Aug. 14, 2007, which claims to benefit ofU.S. Provisional Application No. 60/837,669 filed on Aug. 15, 2006.Application Ser. Nos. 12/377,501, PCT/US2007/075901 and Ser. No.12/377,501 are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for producing biopolymernanoparticles and in particular, starch nanoparticles.

2. Description of the Related Art

U.S. Pat. No. 6,677,386 (which corresponds to WO 00/69916) describes aprocess for producing biopolymer nanoparticles, which in one form arestarch nanoparticles. In the process, the biopolymer is plasticizedusing shear forces, and a crosslinking agent is added during theprocessing. After the processing, the biopolymer nanoparticles can bedispersed in an aqueous medium. One version of the process results instarch nanoparticles which are characterized by an average particle sizeof less than 400 nanometers.

U.S. Pat. No. 6,677,386 notes that the nanoparticles can be used as amatrix material wherein the matrix material may be a film-formingmaterial, a thickener, or a rheology modifier, or an adhesive or anadhesive additive (tackifier). The nanoparticles or dispersions thereofmay also be used for their barrier properties, as a carrier, as a fatreplacer, or as a medicament for mitigating dermal disorders. Furtherexamples of applications for the nanoparticles or dispersions thereofare in the paper-making and packaging industry, or in agriculture andhorticulture. The nanoparticles can also be used as excipients orcarriers in medicines, where they may be complexed or covalently coupledto active substances such as slow-release drugs. The nanoparticles canalso be processed into a foam at relatively high density.

Other uses of the nanoparticles of U.S. Pat. No. 6,677,386 can be foundin: (i) U.S. Patent Application Publication No. 2004/0011487 whichdescribes the use of the starches as a wet-end additive in papermakingpulp slurry, or applied to the surface of the paper as a surface sizingagent; (ii) U.S. Pat. No. 6,825,252 which describes the use of thestarches in a binder in a pigmented paper coating composition; (iii)U.S. Pat. No. 6,921,430 which describes the use of the starches inenvironmentally friendly adhesives; and (iv) U.S. Patent ApplicationPublication No. 2004/0241382 which describes the use of the starches inan adhesive for producing corrugated board. The disclosure of thesepatents and applications, and of all other publications referred toherein, are incorporated by reference as if fully set forth herein.

It can be seen that the nanoparticles prepared by the process of U.S.Pat. No. 6,677,386 have numerous uses. Furthermore, additional uses willlikely be found in the future. As a result, the demand for largerquantities of the biopolymer nanoparticles will continue to increase.However, there are concerns that the process described in U.S. Pat. No.6,677,386 may not be able to keep up with the increasing demand for thebiopolymer nanoparticles.

Therefore, there is a need for an improved process for producing largerquantities of biopolymer nanoparticles and in particular, starchnanoparticles.

SUMMARY OF THE INVENTION

The foregoing need for an improved process for producing a biopolymernanoparticles product is met by the present invention. In the process,biopolymer feedstock and a plasticizer are fed to a feed zone of anextruder having a screw configuration such that the biopolymer feedstockis processed using shear forces in the extruder, and a crosslinkingagent is added to the extruder downstream of the feed zone. An extrudateis foamed through an extrusion die.

In one aspect of the invention, the biopolymer feedstock and theplasticizer are added separately to the feed zone. In another aspect ofthe invention, the extruder has single flight elements in the feed zone.In still another aspect of the invention, the extruder has an upstreamsection, a downstream section, and an intermediate section between theupstream section and the downstream section, and the temperatures in theintermediate section are kept above 100° C.

In yet another aspect of the invention, the screw configuration includestwo or more steam seal sections wherein each steam seal section has anupstream pressure generating section and an adjacent downstream mixingsection. Each upstream pressure generating section has a forwardconveying flight, and each downstream mixing section has a reverseflight. In still another aspect of the invention, shear forces in afirst section of the extruder are greater than shear forces in anadjacent second section of the extruder downstream of the first section,and the crosslinking agent is added to the second section of theextruder. In yet another aspect of the invention, shear forces in afirst section of the extruder are greater than shear forces in a postreaction section of the extruder downstream of the first section whereinthe post reaction section is located in a position in which acrosslinking reaction has been completed, and wherein water is added inthe post reaction section.

The biopolymer feedstock may be starch. Non-limiting examples of thestarch include potato starch, wheat starch, tapioca starch, cassavastarch, rice starch, corn starch, waxy corn starch, and any otherstarches. In one example, the nanoparticles are formed from a highamylopectin based starch (>95% amylopectin, <5% amylose).

The plasticizer may be selected from the group consisting of water,alcohols, and mixtures thereof. The plasticizer may be selected from thegroup consisting of water, polyols, and mixtures thereof.

The crosslinking agent may be selected from dialdehydes andpolyaldehydes, acid anhydrides and mixed anhydrides (e.g. succinic andacetic anhydride) and the like. Suitable dialdehydes and polyaldehydesare glutaraldehyde, glyoxal, periodate-oxidized carbohydrates, and thelike. The crosslinking agent may also be selected from conventionalcrosslinkers such as epichlorohydrin and other epoxides, triphosphates,and divinyl sulphone. The crosslinking reaction may be acid-catalyzed orbase-catalyzed.

A process according to the invention may achieve complete or nearcomplete gelatinization of the biopolymer feedstock. Advantageously, theprocess achieves removal of virtually all of any native biopolymer (e.g.starch) crystalline structure before the crosslinking reaction such thatthe crystalline structure is not detectable using standard techniques(e.g. cross-polarization microscopy). A process according to theinvention meets the challenge of removing native biopolymer (e.g.starch) crystalline structure at higher production rates such as 1metric ton per hour.

A process according to the invention also increases control over theviscosity of a colloidal dispersion of the nanoparticles. The biopolymernanoparticles form a polymer colloid upon dispersion in water, and theextrusion conditions of the present invention have an effect on theviscosity of these colloidal dispersions.

Polymer colloids are impressive in terms of their ability to form highsolids dispersions in water of a relatively high molecular weightpolymer, typically from 40 to 65% solids (the theoretical maximum being72%, which has in fact been achieved for special emulsions with broadparticle size distributions). Yet these dispersions still have amoderately low viscosity (e.g. 500-2500 cps; note that cps=mPa·$). Thesame polymer dissolved in a solvent would typically have a very highviscosity at 10-15% solids (upwards from 5000 cps).

Whereas the viscosity of a polymer in solution is directly proportionalto the molecular weight of that polymer, the viscosity of colloidalemulsions is not. It is independent of molecular weight, and determinedlargely by the number of particles, the size of the particles, and thedistribution of particle size.

The relationship between extrusion conditions of the present inventionand the colloidal dispersion viscosity of the biopolymer (e.g. starch)nanoparticles is not intuitive. The cross-linked nanoparticles producedby the extrusion process of the invention contain a small fraction(<−1%) of uncrosslinked biopolymer (e.g. starch) that acts as a stericstabilizer for the nanoparticles. Such a steric stabilizer is commonlyknown as “protective colloid”, to those skilled in the art. Thebiopolymer (e.g. starch) polymer fragments that serve as the protectivecolloid for the biopolymer (e.g. starch) nanoparticle dispersions areformed as a results of the reactive extrusion process, and areresponsible for the advantageous shear-thinning rheological propertiesof the aqueous nanoparticle dispersions as well as the unexpected andextremely high shear stability observed for this system. The shear ratein the nip of a corrugating roll is about 20,000 s-1 (very high shear),while it is several million s-1 (extremely high shear) in high speedpaper coating applications. Starch nanoparticle dispersions producedaccording to the invention are therefore very well suited as high solidscolloids in corrugating and paper coating applications.

While different extrusion conditions lead to differing viscosities, theyall contain similar sized cross-linked nanoparticles that have highmolecular weight (due to the crosslinks), but in addition contain theprotective colloid that can have lower or higher molecular weightdepending on the aggressiveness of the extrusion conditions. A processaccording to the invention can manipulate the molecular weight of theprotective colloid formed in-situ with the starch nanoparticles.

These and other features, aspects, and advantages of the presentinvention will become better understood upon consideration of thefollowing detailed description, drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an example apparatus suitable for use in theprocess of the invention.

FIG. 2 is a schematic of a packaging system suitable for use in theprocess of the invention.

FIG. 3 shows various screw configurations used in the Examples belowthat serve to illustrate the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an apparatus 20 suitable for use in the process of theinvention. The apparatus 20 can be used to produce the biopolymernanoparticles of U.S. Pat. No. 6,677,386. The apparatus 20 includes atwin screw extruder 22 having eleven extruder barrels 1 to 11 with endflanges by which the barrels 1 to 11 are detachably joined to each otherto create two overlapping bores for receiving the two extruder screws.The apparatus 20 also includes an extruder die 23 with an associatedexhaust hood 23 h.

Barrel 1 has an inlet 24 for receiving a biopolymer (starch in FIG. 1)and plasticizer (glycerol in FIG. 1). The inlet 24 receives dry starchfrom a feed hopper 26 by way of a feed conduit 28. The inlet 24 receivesglycerol from a plasticizer tank 32 by way of a feed conduit 34 thatincludes a feed pump 36 and a Micro-Motion brand mass flow meter 38.Other non-limiting examples of biopolymers that can be processed in theapparatus 20 include other polysaccharides such as cellulose and gums,as well as proteins (e.g. gelatin, whey protein). The biopolymers may bepreviously modified, e.g. with cationic groups, carboxy-methyl groups,by acylation, phosphorylation, hydroxyalkylation, oxidation and thelike. Other non-limiting examples of plasticizers that can be used inthe apparatus 20 include water and in addition to glycerol other polyolssuch as ethyleneglycol, propyleneglycol, polyglycols, sugar alcohols,urea, citric acid esters, etc.

Barrel 3 has an inlet 42 for receiving water. The inlet 42 receiveswater from a water source 44 by way of a feed conduit 46 and a feedconduit 47 which includes a feed pump 48 and a Micro-Motion brand massflow meter 49.

A linking barrel 50 between barrels 7 and 8 has an inlet 52 forreceiving a crosslinker (glyoxal in FIG. 1) and purge water. Inlet 52receives glyoxal from a crosslinker tank 53 by way of a feed conduit 54and a feed conduit 55 that includes a feed pump 56 and a Micro-Motionbrand mass flow meter 57. Inlet 52 receives purge water from watersource 44 by way of feed conduit 46 and a feed conduit 58 that includesa feed pump 59 and a Micro-Motion brand mass flow meter 61. Barrel 10may also receive water from feed conduit 58. Other non-limiting examplesof crosslinkers that can be used in the apparatus 20 include dialdehydesand polyaldehydes, acid anhydrides and mixed anhydrides (e.g. succinicand acetic anhydride), periodate-oxidized carbohydrates, and the like.Suitable dialdehydes are glutaraldehyde and glyoxal. The crosslinkingagent may also be selected from conventional crosslinkers such asepichlorohydrin and other epoxides, triphosphates, and divinyl sulphone.The crosslinking reaction may be acid-catalyzed or base-catalyzed.

Referring now to FIG. 2, there is shown a packaging system 70 of theapparatus 20. The packaging system 70 includes a belt conveyer 72 thatreceives extrudate from the extruder die 23. An appropriate conveyervent system 73 is also provided. The belt conveyer 72 transfersextrudate to a hammer mill 74 for milling the extrudate into smallersized particles. The milled extrudate is then packaged in boxes 78 (orbags or supersacks or bulk trucks or railcars as may be required). Anappropriate mill vent system 82 is also provided to capture fineparticulate matter from the hammer mill 74. As an alternative to thestrand and belt conveyor, a dry hot face cutter and pneumatic conveyorhave been used to cool and convey the product to a mill.

The present invention provides a process that has a unique sequence ofunit operations and a unique process control scheme which permits themanufacture of the biopolymer nanoparticles of U.S. Pat. No. 6,677,386at production rates of greater than or equal to 1.0 metric tons per hourof dry product on a modified ZSK-92 Mega compounder. (A ZSK-92 Megaco-rotating twin screw compounder is available from Coperion Werner &Pfleiderer GmbH & Co., Stuttgart, Germany.) Based on volumetric scaleup, rates of 3 metric tons/hour are anticipated on a ZSK-133 at 500 rpm.Nevertheless, similar results can be obtained on other brands and modelsof extruders by designing their screw configurations.

It is the combination of unit operations used in a process according tothe invention that provides advantages over prior processes. In Examples1 through 10 of U.S. Pat. No. 6,677,386, feed rates were 8.4 to 8.8kg/hr of premix including starch, water and glycerol. This compares witha normal production rate of 110 to 160 kg/hr for a commerciallyavailable extruder sold under the designation Berstorff ZE-40 (which wasused in the examples of U.S. Pat. No. 6,677,386). Steam back venting wasnot a problem in the examples of U.S. Pat. No. 6,677,386 because of thelow temperatures and relatively high area to volume which are both as aresult of the scale of the examples of U.S. Pat. No. 6,677,386.

With geometrically similar machines, the surface area scales are basedon the diameter squared and the process volume based on the diametercubed. This means that as the process is scaled up the area to volumeratio decreases proportionally to the diameter; and that the processmust be operable independent of the barrel temperatures. In addition tothe relative scale, the Berstorff ZE-40 extruder has a low volume forits size as a result of a shallow screw configuration. Relative machinevolume is compared by the ratio of the screw diameter to the rootdiameter or outside diameter/inside diameter (OD/ID). On the BerstorffZE-40 extruder, the OD/ID ratio is only 1.24:1. Most current productionmachines have an OD/ID ratio of 1.46:1 and higher. The ZSK-58, -92, and-133 compounder has a volume ratio of 1.55:1. This is important becauseof the floodability of starch resulting in a relatively low effectivebulk density. In order to achieve viable production rates, higher volumeextruders are desirable. For example, a ZSK-133 compounder can increasethe rate up to the 3 metric tons per hour range.

A. Feeding

Starch is a fine powder which is prone to flooding at high screw speedsand specific rates (kg/hr/rpm or mass of product per hour at given rpm).Given the cost competitive nature of the industry, viable rates for theproduction of the nanoparticles are believed to be at least 1 metric tonper hour. In the examples of U.S. Pat. No. 6,677,386, premixing orpreconditioning the starch was used, which made the starch easier tofeed and reduced its tendency to flood the extruder. It is desirable toeliminate premixing as a process operation and to feed the ingredientsdirectly to the extruder (as in feed conduits 28 and 34 of the apparatus20 of FIG. 1).

Higher volume 2D undercut elements have been used to maximize the solidsconveying capacity. It has been discovered that by using the singleflight (SF) elements (see Robert H. Wildi and Christian Maier,“Understanding Compounding”, Hanser Gardner Publications, 1998, pages97-98), and by injecting either water or glycerol (plasticizer) into thefeed inlet 24, much higher rates can be achieved than with the undercutelements.

Some advantageous process details in the extruder feed zone include,without limitation: (i) the feeding of neat starch, (ii) the feeding ofglycerol and/or water to the center of the feed inlet opening to helpsolids conveying and achieve a similar effect as preconditioning thestarch, and (iii) the use of single flight elements which is good forfloodable powders and minimizes steam back-venting that causes the feedzone to plug.

B. Steam Seal

The process must be run with high temperatures in order to achievecomplete gelatinization of the starch at viable production rates wherethe retention time is on the order of 10 seconds or less. Hightemperatures are also used to control the viscosity of the biopolymernanoparticles product when dispersed in water. These temperatures areabove the boiling point of water at atmospheric pressure; therefore,pressure must be maintained in the extruder 22 to keep the water fromflashing to steam. Because the steam is a separate phase, it can readilyflow backwards towards the feed inlet 24. Once in the feed system, itwill condense and wet the starch, causing flow blockages due topartially cooked starch paste in a gravity flow environment.

It has been discovered that a steam seal must be formed with a series ofrelatively mild restrictions as in the screw design depicted in Screw#92-6 of FIG. 3. (One of the two screws is shown for illustrationthroughout FIG. 3 as is normal in the industry.) Screw #92-1 of FIG. 3had steam back-venting in about 45 minutes and Screw #92-3 of FIG. 3with a strong restriction was operable for less than 15 minutes. It isnecessary to balance these restrictions so that the pressure buildingcapability of the screw is greater than the rise in the vapor pressureof the water due to increasing temperature. Screw #92-1 of FIG. 3 usedrelative mild restrictions: neutral kneading blocks, while that in Screw#92-3 of FIG. 3 used a very strong restriction: a reverse conveyingelement. With the successful Screw #92-6 of FIG. 3, a balance wasachieved by using a series of moderate restrictions, each proceeded byenough pumping and mixing to fill the flights and overcome therestriction.

When the temperatures in the process exceed 100° C., steam seals arenecessary to prevent water from flashing to steam and back venting tothe feed opening. This is done by gradually increasing the pressure inthe extruder faster than the vapor pressure of water increases due tothe increasing temperature used to cook and break down the starch forviscosity control. For example, at 200° C., the absolute vapor pressureof pure water is 1.57 megapascals (i.e. 1.47 megapascals gauge or 213psig). Seals are formed by using a restriction which must be overcome bya forward pumping action. Seals are influenced by the flight fillage inthe extruder with higher specific rates normally resulting in a morerobust seal to a point where the flights become too full for pressuregeneration.

It has been found that if a series of moderate seals are used, thepressure in the extruder can be increased gradually. The effect ofprogressive seals are cumulative. If too strong of a seal is used suchthat the energy and resultant temperature/pressure increase necessary toovercome it is greater than the pressure in the extruder before it,steam will form and back vent. The seals are formed by a combination ofa restriction proceeded by enough forwarding elements to more thanovercome it. In the successful example, three Left Hand (reverse)Kneading Blocks (LKB) are used to generate the steam seal. When pressureis being generated, the flights or kneading blocks will be full. It isimportant that the forward pumping is sufficient to overcome the steampressure increase due to temperature increases in each mixing section.Each mixing section is proceeded by conveying to insure that there isadequate pressure generation. By using a series of such mixing andpressure generation sections, the starch can be heated to increase therate of gelatinization and to control the product viscosity withoutsteam back venting. It is preferred that this be done with mixingsections such as kneading blocks to keep the starch well mixed andeliminate small regions of un-wetted starch that are akin to makinggravy with lumps in it. If these are allowed to form, they will notbecome gelatinized nor subsequently react with the crosslinker and willadversely affect the dispersion viscosity and long term stability ofaqueous dispersions of the product.

The design of the first mixing section/seal is very crucial becausesignificant pressure cannot be generated in the solids conveyingpreceding it. It must be strong enough to initiate the gelatinization(i.e., transition from solid to thermoplastic melt) of the feedstockwithout generating excessive steam pressure. This can be done withtraditional forward and reverse KB combinations or with the Eccentrictri-lobe kneading blocks.

Some advantageous process details regarding the steam seal include,without limitation: (i) the use of progressive seals to eliminate steamback-venting because one strong restriction causes back venting; (ii)the building of pressure faster than the vapor pressure of water to stopback venting; and (iii) the ability to go to higher production rates.

C. Gelatinization

It has been demonstrated that complete gelatinization of the starch isnecessary for the viscosity stability of aqueous dispersions of thebiopolymer nanoparticles product. Residual ungelatinized starchincluding “ghosts” fragments of starch granules and partiallygelatinized starch) will cause a dispersion to gel overnight or in amatter of days on the shelf. The degree of gelatinization can beanalyzed with cross-polarization microscopy. At high rates typical ofmodern extrusion operations, this is very difficult because of therelatively short residence time in the initial mixing zone prior to thecrosslinking reaction zone.

It has been discovered that by using a relatively strong, high shearinitial mixing section with minimal back-flow, complete gelatinizationcan be achieved at high rates. After this high shear section, a seriesof lower shear mixing sections are used to provide mixing, furtherheating, and residence time for the “cooking” of the starch. Asdiscussed above, these are also designed to form a steam seal.

Some advantageous process details to achieve near completegelatinization include, without limitation: (i) water injection at thefeed inlet to plasticize the starch and control product viscosity, (ii)the use of a strong initial kneading zone to avoid residualungelatinized starch including “ghosts”, and (iii) the use ofprogressive seals to eliminate steam back-venting.

D. Reaction

The crosslinking reactant (e.g. glyoxal) is injected to the extruder 22in a moderate to low shear mixing zone designed to provide gooddistributive mixing of the low viscosity liquid into the extremely highviscosity starch paste. This is done to eliminate pooling of thecrosslinking reactant as a separate phase and to achieve distribution inthe starch paste as quickly as possible for a consistent reaction.Although this is extreme in extruders, this is somewhat analogous toadding water to a bread dough, or adding milk to a very thick pancakebatter. After the initial mixing, a series of conveying and mixingsections are used to allow time and mixing for completion of thereaction.

It has been discovered that the crosslinking reactant of the process ofU.S. Pat. No. 6,677,386 should be added after the very high shear zonesused for gelatinization of the starch.

Some advantageous process details to achieve homogeneous reactioninclude, without limitation: (i) glyoxal injection over mixing elementsto eliminate “pooling”, and (ii) the use of staged mixing zones withmild mixing after glyoxal injection, i.e., dividing and combining, notshearing, with good retention time.

E. Post Reaction Conditioning

It has been found that because of the relatively high temperatures ofthe melt phase in the extrusion process (up to 210° C.) used to controlthe final product viscosity when dispersed in water, steam blowing outthe extruder die 23 can be a significant problem affecting both theoperability of the process and consistency of the product quality.Without the proper process, pressure and temperature in the extruderbuild up until it overcomes the die restriction and then literallyempties the end of extruder in a surge or blow out. This flashing coolsthe end of the extruder; and as a result has an effect on the productdispersion viscosity. The net result is a cycle in the viscosity of theinstantaneous discharge and the final product becomes a blend of avariable production.

This problem is overcome by the addition of a very strong seal at theend of the reaction zone to achieve a controlled throttle of the steampressure followed by a post reaction conditioning zone where additionalwater can be added to the product to control the behavior of theextrudate and the bulk properties of the product without uncontrolledaffects on the dispersion viscosity. The strong seal eliminates coolingin the reaction zone. As with the crosslinker (e.g., glyoxal), the postreaction water is injected to the extruder 22 in a moderate to low shearmixing zone designed to provide good distributive mixing of the lowviscosity liquid into the very high viscosity paste.

The post reaction zone is also used to generate the pressure necessaryto pump the product through the die 23.

Some advantageous process details in the post reaction zone include,without limitation: (i) the use of a strong seal to control flashing andeliminate cooling in the reaction zone, (ii) the use of water injectionover mixing elements to eliminate pooling, (iii) the use of waterinjection to control conveyor handling and to control bulk properties ofthe product, and (iv) the application of sufficient pressure to overcomethe restriction of the extruder die to insure continuous pumping to theextruder die.

F. Die Restriction

The die 23 must be designed to generate adequate back pressure tocontrol flashing/cooling in post reaction zone and to minimize surging.It also is used to allow controlled foaming of the extrudate due toflashing of water to steam.

Control of the foaming is very important to the product. Too muchfoaming and the product bulk density is low resulting in extra shippingexpenses. If there is inadequate foaming, it is difficult to rapidlycool and dry the product quickly, and the hard granules that are formedare difficult to disperse in water for the end application.

Thus, some advantageous process details for the extruder die include,without limitation: (i) the use of back pressure to controlflashing/cooling in the post reaction zone and to minimize surging, and(ii) good surface area generation by foaming extrudate noodles,effective cooling and drying by flashing steam, and improved“dispensability” in water because of foaming.

Process Control

Starch is a bio-based feedstock and can vary from lot to lot. Processcontrol is necessary to manipulate the viscosity of the biopolymernanoparticles product in a dispersion for a consistent product. It isalso desirable to produce different viscosity products for variousapplications. It has been discovered that the quantity of water added tothe extruder can be used for such purposes. In the process of theinvention, water is injected in two locations: (1) upstream, beforegelatinization; and (2) downstream, after the crosslinking reaction iscomplete.

A. Upstream Water and Viscosity Mechanism

The first water injection is used as the primary viscosity controlagent. The principal mechanism that affects the dispersion viscosity isdegradation of the starch in the process of producing the biopolymernanoparticles. This can be due to mechanical/shear forces and/or due tothermal degradation of the starch. Evidence from numerous studiesindicates that the thermal effects are more significant. In scale upevaluations without post reaction conditioning, an excellent correlationwas found between the temperature of the extrudate and the dispersionviscosity. In subsequent evaluations on a ZSK-25 mm bench scale twinscrew extruder where surface area and heat transfer can be used to allowvery high specific mechanical energy (SME) and therefore shear inputs tothe product independent of the paste temperature, the controlling factorwas temperature, not shear. In other words, higher barrel temperaturescaused lower SMEs and higher in-process temperatures resulted in lowerdispersion viscosities.

B. Downstream Water

The second (downstream) water is used predominantly to control thehandling characteristics of the product in the die 23 and in the postextruder handling/packaging system 70 by cooling, increasing themoisture content and reducing the foaming of the extrudate. Downstreamwater has a slight effect on the viscosity; however, it is much lessthan that of the upstream water and can be compensated for by minoradjustments of the upstream quantity.

Thus, some advantageous process details to improve dispersion viscosityinclude, without limitation: (i) increasing upstream water to decreasethe product viscosity in a dispersion (and vice versa) because water isa plasticizer in the extruder and is used to control the amount of workinput by the screws, and after the extruder, the water evaporates andtherefore its plasticizing effects are in the extruder only, (ii)increasing downstream water for less blowing and surging at the extruderdie, a more rubbery/less friable noodle, and higher product moisturecontent (and vice versa), (iii) recognizing that downstream water hasmuch less of an effect on the viscosity than upstream water andtherefore when changing downstream water, a much smaller, oppositechange in upstream water should be made to maintain the viscosity, (iv)increasing screw speed to decrease the product viscosity in a dispersion(and vice versa), and (v) increasing barrel temperature to decrease theproduct viscosity in a dispersion (and vice versa).

EXAMPLES

The following examples serve to illustrate the invention and are notintended to limit the invention in any way.

Specific feed and run conditions are listed in Table A and Table B.

Examples 1 and 2

Examples 1 and 2 represent the initial scale up from a ZSK-58 extruderto a ZSK-92 twin screw extruder. See Screw #58-1 and 92-1 in FIG. 3. Oneextra barrel of solids conveying was used on the 92 mm extruder becausean eight barrel configuration was not available. Because Screw #58-1 ofFIG. 3 was deemed too strong (meaning this screw design put too muchenergy into the product), three left hand kneading blocks were removedfrom the gelatinization zone and one from the reaction zone for Screw#92-1 of FIG. 3. Also, an extra water injection was added along with theglyoxal (crosslinker) solution to allow evaluation of the effects of itsconcentration. As received, the glyoxal solution is 40% active in water.At two parts of glyoxal, this is equivalent to a base or minimum of 3parts of water with it if no additional water is used. (See Table A.)

On startup, a preliminary gelatinization experiment without a die wasrun at 726 kg/hr and 300 rpm for a specific rate of 2.42 kg/hr/rpmwithout flooding the feed. This compares to a specific rate equivalentof 1.26 kg/hr/rpm for the ZSK-58 extruder realized at 92 mm. The reasonfor being able to operate with a higher flight fillage is the fact thatthe starch was preconditioned in Example 2 by mixing it with theglycerol off-line in a ribbon blender: thus improving its feedingcharacteristics. Based on subsequent results with this screw, such ahigh specific rate is not sustainable because of buildup filling in theundercut, thus decreasing both the volume and conveying efficiency ofthe two-diameter undercut elements (SKs). For comparison of the ZSK-58extruder to the ZSK-92 extruder, a 3.99:1 volumetric scale up factor isused. Therefore, a specific rate of 0.316 on the ZSK-58 extruder inExample 1 would be equivalent to 1.26 on the ZSK-92 extruder.

Screw #92-1 of FIG. 3 was operated at a flight fillage factor of 1.34 to1.40 kg/hr/rpm and a slightly lower screw speed than Screw #58-1 of FIG.3. There was no steam back-venting on the ZSK-58 extruder; however, withScrew #92-1 of FIG. 3, it would shut the line down after only about 15minutes of continuous operation.

Example 3

Example 3 is similar to Example 2 but represents a reduction in thequantity of water added downstream with the glyoxal (crosslinker) todetermine the effect on the product viscosity. The extra water wasreduced from 3.0 to 2.1 parts. The screw speed for Example 3 was 5%higher than that for Example 2. There was not a significant effect onthe end product viscosity (measured as a standard 25% solids aqueousdispersion at 25° C. and 100 rpm on a Brookfield viscometer). Morenotably, viscosity was slightly higher even though the SME was about 10%higher in Example 3 due to the water and extruder rpm differences. Inprevious work, higher shear rates have resulted in lower dispersionviscosities. This is contrary to that and demonstrates that there areother controlling factors that affect viscosity. As with Example 2,steam back venting shut the line down after a short time.

Example 4

Example 4 was run on Screw #92-2 of FIG. 3. The differences with thisscrew vs. 92-1 of FIG. 3 were the replacement of the 2D pitch SK(undercut elements) in the feed zone with 1.5 D normal elements; thereplacement of a neutral kneading block with a left kneading block nearthe end of Barrel 6; the replacement of the left hand restriction inBarrel 7 with lower shear distributive mixing; a modification of thedistributive mixing for the glyoxal (crosslinker) addition; and anadjustment of the mixing in the reaction zone.

The change in Barrel 6 was an attempt to help keep the steam from backventing. The SKs were replaced because the undercuts were filling upwith hard starch effectively making them normal 2D conveying elements.The modifications in the glyoxal mixing and reaction mixing were becauseit was found that the unwiped slots in the slotted mixing element werefilling up with starch and rendered ineffective. They were replaced withnarrow disk forwarding kneading blocks.

In the initial run with this screw in Example 4, the flood feeding limitwas determined to be 1.35 kg/hr/rpm. When compared with Example 2 whichwas run with the same formulation and a higher screw speed butequivalent specific rate, the resultant SME was higher and thedispersion viscosity was lower as a result of the extra mixing.

Example 5

Example 5 was run on Screw 92-2 of FIG. 3 at a higher screw speed vs.Example 4 to move away from the point of impending feed flooding. Theresult was an increase in the SME and a reduction in the standard 25%solids dispersion viscosity. The line was operated at these conditionsfor 40 minutes before it was shut down due to steam back venting.

Example 6

Example 6 was run on Screw 92-3 of FIG. 3. This screw had left handconveying elements replacing the first two left hand kneading blocks inScrew 92-2 of FIG. 3. This was an attempt to achieve a more effectivesteam seal. The seal was too strong, forcing the steam backwards fromthe first kneading section resulting in plugging of the feed section.The line could not be operated long enough to line out and obtain arepresentative sample.

All subsequent examples (Examples 7-10) were run on Screw 92-6 of FIG.3.

Examples 7 and 8

Examples 7 and 8 are process control examples. Screw 92-6 had a balancedand effective steam seal in the gelatinization zone and had the additionof a post reaction seal followed by a conditioning zone to control theproduct behavior at the die independently of the reaction. Also, toeliminate the need for off-line preblending, it used single flight feedelements and glycerol addition to the center of the feed opening justabove the screws.

All work on this configuration has been consistently run at specificrates of −2 kg/hr/rpm; representing a major improvement over theprevious screw designs. When compared with Example 5, the rate at agiven screw speed is virtually doubled. This is because of the combinedeffect of glycerol injection point and the single flight elements.

The differences between Examples 7 and 8 are the quantities of waterinjected before gelatinization and in the post reaction zone. This wasdone to produce two different dispersion viscosity products, Example 7at 125 mPa·s and Example 8 at 200 mPa·s. The higher viscosity was madeby increasing the upstream water from 0.8 to 11.4 parts. At the higherupstream water loading in Example 8, the post reaction water was notneeded to control the discharge.

Success on the 92-6 screw was based upon the design changes, and inaddition to this, the addition of single flight elements to allow forglycerol addition in the feed zone.

Example 9

The effect of operating at higher rates with proportionally higher screwspeeds for a constant specific rate is demonstrated in Example 9 vs.Example 8. The rate was almost doubled to 1.1 metric tons per hour withno changes other than the screw speed. The product dispersion viscositywas slightly lower which can be compensated for by a slight decrease inthe water loading. Based on extrapolation of this data, 733 rpm would benecessary for 1.5 metric tons per hour. The Mega compounder that hasbeen used in this work can be run at that speed.

Example 10

Example 10 demonstrates the starch feed uptake improvement by additionof glycerol (plasticizer) to the center of the barrel feed opening justabove the screws. The extruder was lined out at the same conditions aswith Example 7 and then the glycerol was turned off. Flooding of thefeed occurred almost immediately and a representative sample was notobtained. The difference in rates for Examples 7 and 10 is the glycerol.

This effect was replicated in some independent work on a ZSK-58 extruderusing a screw that is geometrically similar to Screw #92-6 of FIG. 3.Flooding was almost immediate when the glycerol to the feed opening wasstopped. However, when water was added to the feed opening in place ofthe glycerol, the rates were sustainable.

The data for Examples 1-10 is shown in Tables A and B below. Withrespect to the barrel temperature profiles presented in Table B, highertemperatures were used in Example 1 on the 58 in order to run more of anadiabatic system for better scalability. In Example 6 with Screw 92-3, alower set point was used for Barrel 5 in an attempt to have an effect onthe steam seal. With Screw 92-6, Examples 7-10, the last two barrels forthe post reaction conditioning zone were cooler to aid in the process.

TABLE A Example No. 1 2 3 4 5 6 7 8 9 10 Screw Used 58-1 92-1 92-1 92-292-2 92-3 92-6 92-6 92-6 92-6 (See FIG. 3) Formulation (Parts per 100Dry Starch) Glycerol 10.0 10.0 10.0 10.0 10.0 10.0 10.7 9.9 9.6 NoneGlyoxal 2.0 2.0 2.0 1.9 1.9 1.9 2.0 2.0 2.0 2.0 Upstream Water 0.0 1.11.0 1.2 1.2 1.2 0.8 11.4 11.8 0.8 Water with 3.0 6.0 5.1 6.1 6.3 6.3 3.13.1 3.0 3.1 Glyoxal Post NA NA NA NA NA NA 9.0 0.0 0.0 9.0 ReactionWater Glycerol Brl 2 Pre- Pre- Pre- Pre- Pre- Brl 1 Brl 1 Brl 1 NoneAddition Blend Blend Blend Blend Blend Rate (kg/hr) 178 698 698 410 410410 617 617 1117 562 Active Feed Extruder rpm 564 500 523 303 398 398310 303 543 303 Specific Rate 0.316 1.40 1.34 1.35 1.03 1.03 1.99 2.042.06 1.85 ((kg/hr)/rpm) SME (J/g) 1172 794 886 846 1040 1075 1053 944863 NA Run Time No ~15 mins. 22 mins. Flooded 40 mins. 15 mins. No No NoFlooded re Steam Limit Limit Limit Limit Dispersion 125 147 156 113 101No 123 200 176 No Viscosity [mPa · s] Sample Sample

TABLE B Barrel Temperature Profiles Example No. 1 2 3 4 5 6 7 8 9 10Screw Used 58-1 92-1 92-1 92-2 92-2 92-3 92-6 92-6 92-6 92-6 (See FIG.3) Temperature Set Points (° C.) Barrel 1 Full Cooling Barrel 2 50 10 1010 10 10 10 10 10 10 Barrel 3 120 10 10 10 10 10 10 10 10 10 Barrel 4120 71 71 49 49 49 49 49 49 49 Barrel 5 140 121 121 121 121 49 121 121121 121 Barrel 6 180 121 121 121 121 121 121 121 121 121 Barrel 7 180121 121 121 121 121 121 121 121 121 Barrel 8 180 121 121 121 121 121 121121 121 121 Barrel 9 160 160 160 160 160 121 121 121 121 Barrel 10 93 9393 93 Barrel 11 93 93 93 93 Die 180 124 124 149 149 149 149 149 149 149

Therefore, it can be seen that the invention provides an improvedprocess for producing biopolymer nanoparticles.

Although the invention has been described in considerable detail withreference to certain embodiments, one skilled in the art will appreciatethat the present invention can be practiced by other than the describedembodiments, which have been presented for purposes of illustration andnot of limitation. For example, it is possible to achieve the same unitoperation described herein with different element configurations anddifferent brands and models of twin screw extruders. Therefore, thescope of the appended claims should not be limited to the description ofthe embodiments contained herein.

1. A process for producing a biopolymer product, the process comprising:feeding biopolymer feedstock and a plasticizer to a feed zone of anextruder having a screw configuration such that the biopolymer feedstockis processed using shear forces in the extruder; and wherein the screwconfiguration includes two or more steam seal sections,each steam sealsection having an upstream pressure generating section and an adjacentdownstream mixing section.
 2. The process of claim 1 wherein: eachupstream pressure generating section has a forward conveying flight, andeach downstream mixing section has a reverse flight.
 3. The process ofclaim 1 wherein: the process has a production rate of greater than orequal to 1.0 metric tons per hour of product.
 4. The process of claim 1wherein: the process has a production rate of greater than or equal to3.0 metric tons per hour of product.
 5. The process of claim 1 wherein:the extruder has an upstream section, a downstream section, and anintermediate section between the upstream section and the downstreamsection, and temperatures in the intermediate section are kept above100° C.
 6. The process of claim 1 wherein: shear forces in a firstsection of the extruder are greater than shear forces in an adjacentsecond section of the extruder downstream of the first section.
 7. Theprocess of claim 6 wherein: the crosslinking agent is added to thesecond section of the extruder.
 8. The process of claim 1 wherein: shearforces in a first section of the extruder are greater than shear forcesin a post reaction section of the extruder downstream of the firstsection, the post reaction section being located in a position in whicha crosslinking reaction has occurred, and water is added in the postreaction section.
 9. The process of claim 1 wherein: immediately beforeadding the crosslinking agent, crystalline structure of any nativebiopolymer sampled from the extruder is not detectable usingcross-polarization microscopy.
 10. The process of claim 1 furthercomprising for producing a biopolymer nanoparticles product, the processcomprising: foaming an extrudate through an extrusion die.
 11. Theprocess of claim 10 wherein: the extrudate comprises agglomeratedbiopolymer nanoparticles.
 12. The process of claim 10 furthercomprising: dispersing the extrudate in an aqueous medium.
 13. Theprocess of claim 1 wherein: the biopolymer feedstock is fed to the feedzone without premixing or preconditioning.
 14. The process of claim 1wherein: the biopolymer nanoparticles product forms a polymer colloidupon dispersion in water.
 15. The process of claim 1 wherein: thebiopolymer nanoparticles product includes a protective colloid.
 16. Theprocess of claim 15 wherein: the protective colloid is uncrosslinkedbiopolymer.
 17. The process of claim 1 wherein: the biopolymer productincludes less than about 1% of uncrosslinked biopolymer.
 18. The processof claim 1 wherein: the biopolymer feedstock is starch.
 19. The processof claim 1 wherein: the plasticizer is selected from the groupconsisting of water, alcohols, polyols, glycerol, ethyleneglycol,propyleneglycol, polyglycols, sugar alcohols, urea, citric acid esters,and mixtures thereof.
 20. The process of claim 1 further comprising astep of adding a crosslinking agent to the extruder downstream of thefeed zone.